如何使用实例激活 __str__ dunder 方法并修复错误：“__str__ 返回非字符串（类型 NoneType）”

Question

I have been trying to create a rational class, and add a __str__ dunder method to return the p(numerator), divided by the q(denominator). when i try to print the object rational_1, it only gives me the output of the init method and doesn't return the __str__ dunder method at all. So i tried to use print instead of return, but it raises an error: "__str__ returned non-string (type NoneType)" . How do i print the str and also not raise an error? thanks!

import math
class Rational:
    def __init__(self,p,q):
        self.p=p
        self.q=q
    def __str__(self):
        self.great_dev=math.gcd(self.p,self.q)
        self.p=self.p/self.great_dev
        self.q=self.q/self.great_dev
        print ("{self.p}/{self.q}".format(self=self))
rational_1=Rational(3,60)
print(rational_1)

Answer 1

As the error says, you need to return the new string representation of the class instance you created in the overriden __str__ dunder method.

def __str__(self):
    self.great_dev=math.gcd(self.p,self.q)
    self.p=self.p/self.great_dev
    self.q=self.q/self.great_dev
    # return the updated string representation
    return "{self.p}/{self.q}".format(self=self)

Then the code will expected and the output will be 1.0/20.0